A conventional electro-mechanical speaker requires a cabinet to absorb the back waves from the speaker to prevent them from cancelling with the front waves projecting off of the front of the speaker. In order to produce a wide frequency range, the cabinet must be large enough so that it is not appreciably pressurized as the driver speaker is producing low frequency tones since to do so restricts the driver cone which must move substantial distances to produce the low frequencies. Formulae have been developed to determine the size of the cabinet for the required low frequency response. To simulate live music and its related effects, such as pressure and ambiance, audiable output well below 40 Hz is desirable. Unfortunately, to produce this type of sound, the cabinet must be very large to prevent pressurization and the distortion it can cause.
The prior art relies on several devices to enable the production of low frequency response in reasonably sized cabinets. Probably the most popular are linear pressure relief systems which are known in the trade as Ventri ports or micro ports. It is believed they were developed by John Iverson in the 1960's as "acoustic diodes". An acoustic diode essentially is nothing more than a tapered hole. Air can pass from the smaller end to the larger end of the hole faster than it can pass in the other direction due to the flow characteristics thereof. Such ports allow a great reduction in cabinet size without loss of important low frequency reproduction. There are, however, restrictions on how small the cabinet can be because as the cabinet decreases in volume, an increase in pressure results which eventually causes the ports to whistle. The frequency of this whistle is higher than the fundamental tone, so if the sound level is high enough, the whistling interferes with the low frequency response of the system. Increasing the number of ports does not reduce the pressure sufficiently to stop the whistling. This would seem to indicate that there is a minumum size for a cabinet even if it has a linear venting system. Such a venting system is shown in U.S. Pat. No. 4,119,799 entitled "Critical Alignment Loudspeaker System" by Mark Merlino, a co-inventor hereof. The application shows such ports included in an infinite baffle type speaker system. This type of venting system shoud not be confused with tuned port systems. They allow the back wave to exit the cabinet in phase with the front wave. In most conventional bass reflex speaker systems this gives the effect of low energy by boosting the level around 50 Hz. Extended response below 40 Hz is impossible with this type of system without the added features shown in the referenced application.
Transmission lines and passive pistons can actually increase the low frequency response of a speaker system but control of such complex devices becomes a problem due to inertia, and such system never truly can be linear. Electronic equalization can tailor the low frequency response of the system, but with an increase in noise and distortion due to longer signal path.